A rubber composition for a sidewall of a tire is required to be excellent in steering stability, rolling resistance (heat build-up property), durability, crack growth resistance, resistance to scratch, conductivity, anti-spew-cutting property, extrusion processability, and the like. Particularly a rubber composition for a sidewall of general-purpose tires is required to be excellent in conductivity, anti-spew-cutting property and extrusion processability in good balance and be able to be prepared at low cost.
The conductivity of a tire is a property for discharging static electricity accumulated on a vehicle to a ground, and generally static electricity is discharged through a rim, a clinch, a ply, a sidewall, a belt or a breaker, or a tread.
Without such discharging, static electricity is accumulated on a vehicle and there arises a problem such that a person riding on a vehicle receives an electric shock, a spark occurs when filling fuel to cause a fire, mixing of a noise on a radio in a vehicle during running occurs, or corrosion of a rubber composition of a clinch portion due to an electrically charged rim is accelerated.
A rubber composition comprising a high structure carbon black is known for securing conductivity of a rubber composition for a sidewall. However, a high structure carbon black is expensive as compared with usual carbon blacks, and has a problem with cost for the use for a rubber composition for a sidewall of general-purpose tires required to be able to be produced at low cost.
In the case where it is difficult to secure conductivity of a sidewall, it is known, as a technology for discharging without passing through a sidewall, to secure conductivity of a tire by providing an electrically conductive layer having good conductivity at an inner side of a sidewall as described in Patent Document 1. However, since a rubber composition comprising a carbon black as a main filler is used for the electrically conductive layer, there is a problem in using this rubber composition for general-purpose tires from the viewpoint of deterioration of durability and rolling resistance and increase in production cost due to complexity of the production steps.
Further, as a technology for discharging without passing through a sidewall, it is known to improve conductivity of a ply topping rubber layer for discharging through a rim, a clinch, a ply topping rubber, a belt or a breaker, or a tread. However, since a rubber composition comprising a carbon black as a main filler is used as the ply topping rubber and the ply topping rubber layer is thickened, there is a problem in using this rubber composition for general-purpose tires from the viewpoint of deterioration of heat build-up property, durability and rolling resistance of the ply.
It is said that the spew cutting arises at de-molding during the tire production process and is related to breaking resistance (TB) and elongation at break (EB) of a rubber composition at a high temperature (140° C.). Therefore, in order to prevent lowering of EB and TB at high temperatures, countermeasures are taken, such as large increase in a content of natural rubber in a rubber component and increase in contents of zinc oxide and a mold release agent, but in applying these countermeasures for general-purpose tires, there is problem with cost.
Patent Document 2 describes a rubber composition for a sidewall of a tire which enables the tire to have excellent rolling resistance, abrasion resistance and wet performance and a small electric resistance, by comprising a specific carbon black, silica and a silane coupling agent in predetermined amounts. However, in the rubber composition for a sidewall of a tire of Patent Document 2 comprising a silane coupling agent in a predetermined amount, there is a problem with cost, and neither anti-spew-cutting property nor extrusion processability is considered.